We conducted moment tensor inversion and studied source rupture process for MS=7.9 earthquake occurred in the border area of China, Russia and Mongolia on September 27 2003, by using digital teleseismic P-wave seismo-grams recorded by long-period seismograph stations of the global seismic network. Considering the aftershock distribution and the tectonic settings around the epicentral area, we propose that the MS=7.9 earthquake occurred on a fault plane with the strike of 127°, the dip of 79° and the rake of 171°. The rupture process inversion result of MS=7.9 earthquake shows that the total rupture duration is about 37 s, the scalar moment tensor is M0=0.97×1020 N?m. Rupture mainly occurred on the shallow area with 110 km long and 30 km wide, the location in which the rupture initiated is not where the main rupture took place, and the area with slip greater than 0.5 m basically lies within 35 km deep middle-crust under the earth surface. The maximum static slip is 3.6 m. There are two distinct areas with slip larger than 2.0 m. We noticed that when the rupture propagated towards northwest and closed to the area around the MS=7.3 hypocenter, the slip decreased rapidly, which may indicate that the rupture process was stopped by barriers. The consistence of spatial distribution of slip on the fault plane with the distribution of after-shocks also supports that the rupture is a heterogeneous process owing to the presence of barriers. We conducted a survey of long-period seismograph stations by MS global seismic network. Considering the aftershock distribution and the tectonic settings around the epicentral area, we propose that the MS = 7.9 difficulty occurred on a fault plane with the strike of 127 °, the dip of 79 ° and the rake of 171 °. rupture process inversion result of MS = 7.9 earthquake shows that total rupture duration is about 37 s, the scalar moment tensor is M0 = 0.97 × 1020 N? m. Rupture mainly occurred on the shallow area with 110 km long and 30 km wide, the location in which the rupture initiated is not where the main rupture took place, and the area with slip greater than 0.5 m basically lies within 35 km deep middle-crust under the earth surface. There are the maximum static slip is 3.6 m. There are two distinct areas with slip larger than 2.0 m. We noticed that when the rupture propagated towards northwest and closed to the area around the MS = 7.3 hypocenter, the slip decreased rapidly, which may indicate that the rupture process was stopped by barriers. of spatial distribution of slip on the fault plane with the distribution of after-shocks also supports that the rupture is a heterogeneous process due to the presence of barriers.